I have no personal experience with them, but other TB members have mentioned products that are sold under the brand name "Rolls", and if you check their product lineup, you may find something to suit your requirements.

The cheapest is a DIY passive mixer.
Distortion is impossible with modern resistors.

You can use a rotary switch with diffferent resistor combinations for different mixing ratios.

If You can reuse something as the box, the resistors, jacks (/cords+plugs) and the rotary switch should be around $15.

The input of the SVT should be sensitive enough not to cause any problems with the signal strength.

A mini mixer would be a much better choice, even though a bit more expensive.

Regards
Sam

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I was going to suggest this exactly, but you'd be better off using a blend pot, and you could use an Lm741 opamp wired inverting with a 100k pot in parallel to use as a gain stage post-mix. Wouldn't be passive, but then you could just use it in pedal form. Pedals being what others are suggesting of course.

The blend pot is a good idea, I'd use an emg blend pot and three jacks in a little box. Or wire a dual gang 25KB (linear) pot like this:

If your bass is passive, and all your pedals are tru-bypass, however, this won't work well.

You could also make a Y-lead with a couple of resistors in the plugs. You could make one from scratch or mod one from a store. A couple of 10K resistors will work for 99% of pedals. The resistors go in series with the signal from each chain.